Browsing by Subject "Artificial noise"
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Item Open Access Artificial-noise-aided secure transmission over finite-ınput ıntersymbol ınterference channels(IEEE, 2018-06) Hanoğlu, Serdar; Aghdam, Sina Rezaei; Duman, Tolga M.We propose an artificial noise (AN) injection strategy for securing communication over finite-input intersymbol interference (lSI) channels. The technique relies on injection of colored noise whose power spectral density has the least match with the spectrum of the main channel in a certain sense. By evaluation of an achievable secrecy rate, we demonstrate that the proposed AN injection based solution results in a considerable improvement over the existing approaches, especially when the eavesdropper works at high signal-to-noise ratios (SNRs).Item Open Access Joint precoder and artificial noise design for MIMO wiretap channels with finite-alphabet inputs based on the cut-off rate(Institute of Electrical and Electronics Engineers Inc., 2017) Aghdam, S. R.; Duman, T. M.We consider precoder and artificial noise (AN) design for multi-antenna wiretap channels under the finite-alphabet input assumption. We assume that the transmitter has access to the channel coefficients of the legitimate receiver and knows the statistics of the eavesdropper's channel. Accordingly, we propose a secrecy rate maximization algorithm using a gradient descent-based optimization of the precoder matrix and an exhaustive search over the power levels allocated to the AN. We also propose algorithms to reduce the complexities of direct ergodic secrecy rate maximization by: 1) maximizing a cut-off rate-based approximation for the ergodic secrecy rate, simplifying the mutual information expression, which lacks a closed-form and 2) diagonalizing the channels toward the legitimate receiver and the eavesdropper, which allows for employing a per-group precoding-based technique. Our numerical results reveal that jointly optimizing the precoder and the AN outperforms the existing solutions in the literature, which rely on the precoder optimization only. We also demonstrate that the proposed low complexity alternatives result in a small loss in performance while offering a significant reduction in computational complexity.Item Open Access An overview of physical layer security with finite-alphabet signaling(Institute of Electrical and Electronics Engineers Inc., 2019) Aghdam, Sina Rezaei; Nooraiepour, A.; Duman, Tolga M.Providing secure communications over the physical layer with the objective of achieving secrecy without requiring a secret key has been receiving growing attention within the past decade. The vast majority of the existing studies in the area of physical layer security focus exclusively on the scenarios where the channel inputs are Gaussian distributed. However, in practice, the signals employed for transmission are drawn from discrete signal constellations such as phase shift keying and quadrature amplitude modulation. Hence, understanding the impact of the finite-alphabet input constraints and designing secure transmission schemes under this assumption is a mandatory step toward a practical implementation of physical layer security. With this motivation, this paper reviews recent developments on physical layer security with finite-alphabet inputs. We explore transmit signal design algorithms for single-antenna as well as multi-antenna wiretap channels under different assumptions on the channel state information at the transmitter. Moreover, we present a review of the recent results on secure transmission with discrete signaling for various scenarios including multi-carrier transmission systems, broadcast channels with confidential messages, cognitive multiple access and relay networks. Throughout the article, we stress the important behavioral differences of discrete versus Gaussian inputs in the context of the physical layer security. We also present an overview of practical code construction over Gaussian and fading wiretap channels, and discuss some open problems and directions for future research.Item Open Access Secrecy rates of finite-input intersymbol interference channels(Bilkent University, 2016-10) Hanoğlu, SerdarDue to the broadcast nature of the communication medium, security is a critical problem in wireless networks. Securing the transmission at the physical layer is a promising alternative or complement to the conventional higher level techniques such as encryption. During the past decade, various studies have been carried out which investigate such possibilities in providing secrecy for different scenarios. On the other hand, secrecy over intersymbol interference (ISI) channels has not received significant attention, and much work remains to be done. With this motivation, we focus on secrecy rates of finite-input ISI channels for both fixed and fading channel coefficients. We argue that the secrecy rates of ISI channels can be computed by the forward recursion of the BCJR algorithm. Moreover, by utilizing Markov input distributions for transmission over the ISI channels, achievable secrecy rates can be increased. However, the existing iterative method in the literature to obtain the optimal Markov input distribution is computationally complex as many BCJR recursions are needed. Thus, we propose an alternative solution by introducing a codebook based approach. Particularly, among the existing Markov input distributions in the codebook, we propose to select the one which spectrally matches the main channel. Our numerical results reveal that the proposed low complexity approach undergoes a minimal loss with respect to the existing iterative algorithm while offering a considerably reduced complexity. We also propose injection of artificial noise (AN) to increase the secrecy rates, and show that this is especially useful for moderate and high signal to noise ratio (SNR) values where the use of Markov input distributions is not beneficial. We inject AN to frequencies where the eavesdropper's channel is better than the main channel. We show that this approach significantly increases the secrecy rates compared to the existing methods. Furthermore, we consider the effect of channel state information (CSI) on the secrecy rates, and demonstrate that availability of eavesdropper's CSI at the transmitter is highly beneficial in terms of the achievable secrecy rates.Item Open Access Secure multi-antenna transmission with finite-alphabet signaling(Bilkent University, 2017-12) Aghdam, Sina RezaeiWith the ever-growing demand for services that rely on transmission over wireless networks, a challenging issue is the security of the transmitted information. Due to its open nature, wireless communications is prone to eavesdropping attacks. Typically, secrecy of the transmitted information is ensured with the aid of cryptographic techniques, which are deployed on upper layers of the network protocol stack. However, due to the need for key distribution and management, cryptographic solutions are difficult to implement in decentralized networks. Moreover, the security provided by key based solutions is not provable from a mathematical point of view. Physical layer security is an alternative or complement to the cryptographic techniques, which can resolve the complexities associated with key distribution and management. The basic principle of physical layer security is to exploit the randomness of the communication channels to allow a transmitter deliver its message to an intended receiver reliably while guaranteeing that a third party cannot infer any information about it. Much of the existing research in physical layer security focuses on investigating the information theoretic limits of secure communications. Among different techniques proposed, multiple-antenna based solutions have been shown to exhibit a high potential for enhancing security. Furthermore, Gaussian inputs are proved to be the optimal input distributions in a variety of scenarios. However, due to the high detection complexity, Gaussian signaling is not used in practice, and the transmission is carried out with the aid of symbols drawn from standard signal constellations. In this thesis, we develop several secure multi-antenna transmission techniques under the practical finite-alphabet input assumption. We first consider multipleinput multiple-output (MIMO) wiretap channels under finite-alphabet input constraints. We assume that the statistical channel state information (CSI) of the eavesdropper is available at the transmitter, and study two different scenarios regarding the transmitter's knowledge on the main channel CSI (MCSI) including availability of perfect and statistical MCSI at the transmitter. In each scenario, we introduce iterative algorithms for joint optimization of data precoder and arti ficial noise. We also propose different strategies to reduce the computational complexity associated with the transmit signal design. Moreover, we consider the setups with simultaneous wireless information and power transfer (SWIPT), and propose transmission schemes for achieving the trade-off between the secrecy rate and the harvested power. We demonstrate the efficacy of the proposed transmit signal design algorithms via extensive numerical examples. We also introduce several secure transmission schemes with spatial modulation and space shift keying (SSK). We derive an expression for the achievable secrecy rate, and develop precoder optimization algorithms for its maximization using transmitter side CSI. Furthermore, we introduce a group of secure SSK transmission schemes, which rely on dynamic antenna index assignment over reciprocal channels. Our results reveal that the fundamentally different working principle of SSK opens up new avenues for secure multi-antenna transmission.